Growth in the global N 2 sink attributed to N fertilizer inputs over 1860 to 2000 Chao Wang a , Benjamin Z. Houlton b , Weiwei Dai a , Edith Bai a, ⁎ a Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China b Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA HIGHLIGHTS • A stable isotope model was used to con- strain the soil gaseous N losses. • Soil N 2 emission from natural and crop- land ecosystem was investigated. • Terrestrial soil N budget increased two- fold from 1860 to 2000. • Cropland expansion increased the soil reactive N (N 2 O, NO x , NH 3 and leaching) losses. GRAPHICAL ABSTRACT abstract article info Article history: Received 6 August 2016 Received in revised form 19 September 2016 Accepted 19 September 2016 Available online 14 October 2016 Editor: Jay Gan Cropland expansion and fertilizer applications are among the most important substantial effects of human actions on the global nitrogen (N) cycle. However, questions remain over the fate of anthropogenic N inputs, particularly whether a significant fraction of N-based fertilizers have been lost to inert N 2 or reactive N forms. Here, we combine natural N isotope constraints on the pre-industrial N cycle with global mass-balance modeling to investigate the role of cropland conversion on gaseous N emissions and hydrological N leaching fluxes. We estimate that cropland ex- pansion has been accompanied by N 9-fold increase in N input rates to cropping systems, roughly doubling the base- line N budget of the terrestrial biosphere. As a consequence, approximately 10 times more N is exported from modern croplands to the hydrosphere than in 1860, with a five-fold increase in cropland N gases emission to the atmosphere. Atmospheric NH 3 , NO, N 2 O and N 2 fluxes increased from 8.6, 16.6, 11.7 and 31.9 Tg N yr -1 , respectively, in 1860 to 17.7, 23.6, 15.2 and 39.7 Tg N yr -1 , respectively, by 2000. Thus, the growth in N 2 accounted for ~20% of cropland-driven N losses (dissolved plus gaseous pathways), with the remaining 80% exported as reactive N forms. Although the increase in N 2 emissions has mitigated some of the unwanted side-effects of N fertilizer applications on human health, the economy, and climate change, this inert sink has been unable to keep pace with the increase in N inputs for enhanced food production. Our results imply that, unless new management steps are taken, an increasing fraction of N fertilizers will mobilize to reactive N forms in the global land, air and water systems, thus further accel- erating the negative consequences of human modifications of the N cycle this century. © 2016 Elsevier B.V. All rights reserved. Keywords: Land use change Human activities Biogeochemical cycling Gaseous losses Global nitrogen budget Nitrogen use efficiency Science of the Total Environment 574 (2017) 1044–1053 ⁎ Corresponding author at: Institute of Applied Ecology, Chinese Academy of Sciences, No. 72 Wenhua Road, Shenyang 110016, Liaoning, China. E-mail address: baie@iae.ac.cn (E. Bai). http://dx.doi.org/10.1016/j.scitotenv.2016.09.160 0048-9697/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv